linux-kernel - Re: [PATCH 1/5] KVM: arm64: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX

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Message-ID: <ad3534bc-fe3a-55f5-b022-4dbec5f29798@redhat.com>
Date:   Fri, 12 Nov 2021 15:10:41 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marc Zyngier <maz@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        Andrew Jones <drjones@...hat.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Anup Patel <anup.patel@....com>,
        Paul Mackerras <paulus@...abs.org>,
        Michael Ellerman <mpe@...erman.id.au>, kvm-ppc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] KVM: arm64: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX_VCPUS

On 11/12/21 15:02, Marc Zyngier wrote:
>> I'd like KVM to be consistent across architectures and have the same
>> (similar) meaning for KVM_CAP_NR_VCPUS.
> Sure, but this is a pretty useless piece of information anyway. As
> Andrew pointed out, the information is available somewhere else, and
> all we need to do is to cap it to the number of supported vcpus, which
> is effectively a KVM limitation.
> 
> Also, we are talking about representing the architecture to userspace.
> No amount of massaging is going to make an arm64 box look like an x86.

Not sure what you mean?  The API is about providing a piece of 
information independent of the architecture, while catering for a ppc 
weirdness.  Yes it's mostly useless if you don't care about ppc, but 
it's not about making arm64 look like x86 or ppc; it's about not having 
to special case ppc in userspace.

If anything, if KVM_CAP_NR_VCPUS returns the same for kvm and !kvm, then 
*that* is making an arm64 box look like an x86.  On ARM the max vCPUs 
depends on VM's GIC configuration, so KVM_CAP_NR_VCPUS should take that 
into account.  Or KVM_CAP_NR_VCPUS should have been only for !kvm; but 
the ship for that has sailed.

Paolo

>>> which I'm keen on avoiding. I'd rather have the kvm and !kvm cases
>>> return the same thing.
>> Forgive me my (ARM?) ignorance but what would it be then? If we go for
>> min(num_online_cpus(), kvm_arm_default_max_vcpus()) in both cases, cat
>> this can still go above KVM_CAP_MAX_VCPUS after vGIC is created?
> "min(num_online_cpus(), kvm_arm_default_max_vcpus())" is probably the
> right thing in all cases. Yes, KVM_CAP_NR_VCPUS will keep reporting
> more than the VM can actually support. But that's why we have
> KVM_CAP_MAX_VCPUS, which tells you now many vcpus you can create for a
> given configuration.